Yes, U.S. citizens can technically go on a "closed-loop" cruise (one that starts and ends at the same U.S. port, like Miami to Miami) without a passport. Under the Western Hemisphere Travel Initiative (WHTI), you can instead use a government-issued photo ID (like a driver's license) and an original or certified copy of your birth certificate. However, even if the cruise line allows this, it is strongly discouraged by travel experts and the State Department. If you have an emergency and need to fly home from a foreign port (like the Bahamas or Mexico), you cannot board a plane back to the U.S. without a valid passport book; a birth certificate is only valid for land and sea borders. Furthermore, if the ship is forced to divert to a non-WHTI port due to weather or mechanical issues, you may face significant legal hurdles. For 2026 travel, having a passport remains the "gold standard" for safety and flexibility, ensuring you aren't stranded if your vacation doesn't go exactly as planned.